1. Field of the Invention
This invention relates generally to the field of liquid oxygen storage and delivery systems, and, specifically, to systems and methods to efficiently and effectively operate liquid oxygen storage and delivery systems.
2. Description of the Related Art
A growing number of people suffer from chronic hypoxemia as a result of having a chronic obstructive pulmonary disease (COPD) such as asthma and emphysema, as well as cystic fibrosis, lung cancer, lung injuries, cardiovascular diseases, and otherwise diseased or damaged lungs. Presently, there is no cure for COPD. However, the detrimental impact of chronic hypoxemia is mitigated by the prescription of oxygen therapy in which oxygen enriched gas or pure oxygen is delivered to the airway of the user. The oxygen therapy serves to compensate for the poor function of the patient's lungs in absorbing oxygen.
The delivery of supplemental oxygen may be provided utilizing one of three predominant methods. For non-ambulatory patients, or for use during non-ambulatory periods, oxygen may be provided from a stationary oxygen concentrator, which typically makes use of a pressure swing adsorption system to generate the oxygen. Oxygen concentrators utilizing pressure swing adsorption (“PSA”) systems are advantageous in that they can process ambient air, containing approximately 21% oxygen, and separate that oxygen from the ambient air. Thereby, the patient can be supplied with gas containing higher concentrations of oxygen. While suitable for their intended purpose, oxygen concentrators are generally bulky and require access to a power source, such as an electrical outlet. Thus, oxygen concentrators are ill-suited for portability and are not intended for use with an ambulatory individual.
Compressed oxygen systems are generally prescribed when oxygen is not needed all the time, such as only when walking or performing physical activity. One significant disadvantage of a compressed oxygen system is that because the oxygen is stored under pressure, the tank may be hazardous if dropped. In addition, small portable compressed oxygen tanks are limited in how long they will last depending on the prescribed flow rate and type of tank.
An alternative to compressed oxygen systems is a liquid oxygen (“LOX”) system. Liquid oxygen is advantageous because it occupies a tenth of the space of compressed gaseous oxygen. To maintain a liquid state, however, oxygen must be kept at a relatively cool temperature around 300 degrees Fahrenheit below zero.
A conventional LOX system includes a large stationary LOX storage canister that stays in the home. The conventional system also includes a small, portable delivery apparatus that can be filled from the stationary unit for trips outside the home. Many first generation systems have limited utilization due to the low LOX capacity of the portable delivery apparatus and the administered LOX flow rate. Furthermore, even when not in use, the LOX within the portable delivery apparatus evaporates at a typical rate of one pound per day, thereby emptying the portable delivery apparatus LOX supply over time. Consequently, a disadvantage of a conventional portable LOX system includes the requirement that the user must return home by the end of the day to refill the portable delivery apparatus from the home stationary LOX storing canister.
Many first generation LOX systems provide a constant flow of oxygen to the patient. In these LOX systems, a flow-meter or fixed orifice can provide a desired level of oxygen to the patient at a constant flow rate. Although successful in delivering oxygen therapy at adequate levels, these LOX systems waste a significant amount of oxygen. This is due to the nature of the cycle of pulmonary gas exchange by a patient. Typically, it is only the gas inhaled during the half a second at the beginning of an inspiration that delivers oxygen to the blood stream of a patient. More specifically, it is only the oxygen that reaches the pulmonary alveoli, or spherical outcroppings of the respiratory bronchioles, that is exchanged and received within the blood stream. Therefore, for LOX systems that provide a constant flow of oxygen, the oxygen delivered at times other than the first half second of inspiration is wasted. This is highly significant to portable LOX systems that have a limited capacity of oxygen to supply to the patient.
To limit the amount of oxygen wasted by constant flow LOX system, oxygen conserving devices (“OCD”) have been designed to attempt to interrupt the flow of oxygen in accordance with the patient's breathing cycle. Therefore, these oxygen conservers had to be capable of sensing inspiration by the patient to permit the therapeutic flow of oxygen during the beginning of inhalation and stop the flow of oxygen during the end of inhalation and during exhalation. It should be noted that the terms “oxygen conserving device”, “conserving device”, and “conserver” are used interchangeably.
Oxygen conserving devices are generally of two types, those which operate electronically, and those which operate pneumatically. Each type presents different benefits and disadvantages.
Electronic conservers require a power source, generally a battery, in order to operate, thus necessitating periodic replacement or recharging of the power source. Further, electronic conservers have integrated circuitry that most often has temperature range limitations. Electronic oxygen conservers sometimes have further disadvantages related to durability and cost.
Pneumatic oxygen conservers, however, make use of the pressurized gas and its flow within the conserver to intermittently block the delivery of gas to the person. Therefore, pneumatic conservers generally dispense with the need for power sources and complex electronics. Conventional pneumatic conservers, however, are oftentimes bulkier and generally require more complex gas lines or cannulas in order to operate.
Many conventional pneumatic conservers utilize a dual lumen cannula. Examples of conventional pneumatic conservers and their associated dual-lumen cannulas are disclosed in U.S. Pat. No. 4,044,133 to Myers and U.S. Pat. No. 5,360,000 to Carter. One lumen of the cannula is for supplying oxygen to the person wearing the cannula, whereas the other lumen generally connects to a sensing port on the conserver. The pneumatic conserver generally responds to changes in the pressure in the sensing lumen to provide oxygen to the person during inhalation and to interrupt the flow of oxygen to the person in response to exhalation. Unfortunately, dual lumen cannulas are more expensive, bulkier, and generally not as comfortable to the patient as single lumen cannulas used in electronic conservers and many other medical devices.
Conventional pneumatic conservers suffer from a significant drawback in that while they do aid in preventing the waste of some therapeutic oxygen, they often permit and even contribute to the waste of oxygen. More particularly, conventional pneumatic conserver devices typically operate by sensing negative pressure at the outlet of the conserver. When negative pressure is present at the outlet, corresponding to inhalation by the patient, the sensing valves of the pneumatic conserver will open, causing the control valves to open and permit the flow of oxygen from the output. Thus, when the patient begins inhalation, the control valve will open for the first pulse. If the patient continues to inhale, a negative pressure at the outlet will still be present and the control valves may remain open, or open again, and continue to output oxygen. As only the first half second of oxygen is therapeutic, the oxygen passed through the control valve and output after this first half second is wasted.
As a result of these and other drawbacks, pneumatic oxygen conserving devices have not enjoyed widespread use despite certain advantages of such pneumatic conservers over electronic ones. The various attempts to overcome the drawbacks of pneumatic conservers have had mixed results and have generated their own drawbacks and disadvantages.
There are a number of oxygen delivery systems that have attempted to overcome the drawbacks associated with pneumatic oxygen conserving devices. One such oxygen delivery system is disclosed in PCT application no. PCT/US00/29374 (PCT publication no. WO 01/33135)(“the '374 application”). The commercial name of the product described in the '374 application is the HELiOS®. As identified at the HELiOS® website, www.heliosoxygen.com, the HELiOS® H300 portable LOX delivery unit has a limited capacity for storing a content of liquid oxygen enabling eight to ten hours of usage before the LOX is depleted when the device has a setting of two. The HELiOS® weighs approximately 3.6 pounds when full with LOX, and 2.75 pounds empty.
Another oxygen delivery system with a pneumatic oxygen conserver is the Easymate Liquid Oxygen System sold by Precision Medical, Inc. According to Precision Medical, Inc.'s website, www.precisionmedical.com, the Easymate Liquid Oxygen System is 3.6 pounds and provides a single lumen system that utilizes an oxygen conserving regulator. Furthermore, the Chad Cypress oxygen delivery system sold by Chad Therapeutics, Inc. includes a pneumatic oxygen conserver. All three of these devices, the H300, the Easymate Liquid Oxygen System, and the Chad Cypress, attempt to overcome some of the drawbacks associated with pneumatic oxygen conservers.
While suitable for their intended purposes, the prior art oxygen delivery systems with pneumatic oxygen conservers suffer from many drawbacks. The most significant drawback is that the pneumatic oxygen conservers of these oxygen delivery systems still permit oxygen to be wasted. More specifically, the pneumatic oxygen conservers generally deliver oxygen in a manner inconsistent with the oxygen consumption profiles of a person breathing through a cannula. Thus, the conventional pneumatic oxygen conservers are incapable of conserving oxygen at a desired level.
FIG. 1 provides an illustration of the pulse flow waveform of oxygen in standard liters per minute versus time for certain conventional oxygen delivery systems with conventional oxygen conservers. In FIG. 1, the time value of zero represents the beginning of inhalation by the patient. Oxygen conserving devices are typically triggered by sensing a negative pressure, corresponding to an inhalation by the patient. Therefore, most oxygen conserving devices are configured to trigger at a negative pressure level achieved at some time period after the patient begins to inhale. The pulse flow waveforms shown in FIG. 1, provided in units of Standard Liters Per Minute (“SLPM”), are representative of delivery triggered by a typical inhalation during a patient's respiratory cycle.
Waveform 105 is a plot of the pulse flow of the Chad Cypress oxygen delivery system. As illustrated by waveform 105, the delivery of oxygen to the patient occurs at a relatively low flow rate over a relatively long period of time. As previously discussed, it is generally only the oxygen delivered within the first half second (0.5 seconds) after inspiration (time 0.0 on the graph of FIG. 1) that provides therapeutic benefit. As shown in FIG. 1, oxygen flow profile for the Chad Cypress oxygen delivery system graphed by waveform 105 does not begin until around 0.3 seconds and peaks at only 2 SLPM of oxygen. Furthermore, as shown by waveform 105, the Chad Cypress oxygen delivery system continues to provide oxygen for at least 1.2 seconds after the beginning of inspiration. The majority of the oxygen output by the Chad Cypress oxygen delivery system after 0.5 seconds is wasted. Thus, the pneumatic oxygen conserver of the Chad Cypress oxygen delivery system wastes oxygen by delivering during non-therapeutic periods of the respiratory cycle.
FIG. 1 also provides a plot of the pulse flow of Precision Medical, Inc.'s Easymate Liquid Oxygen System in waveform 110. As shown by waveform 110, the Easymate Liquid Oxygen System does not begin delivering oxygen until at least 0.3 seconds after inspiration. For delivery shown by waveform 110, the Easymate Liquid Oxygen System delivers at a rate of about 10 SPLM initially and steadily decreases from its initial peak rate. Although the Easymate Liquid Oxygen System delivers more oxygen than the Chad Cypress oxygen delivery system before 0.5 seconds after inspiration, it still continues to deliver oxygen after the 0.5 seconds mark. As shown by waveform 110, the Easymate Liquid Oxygen System continues to deliver oxygen until around 0.65 seconds. Thus, the Easymate Liquid Oxygen System wastes oxygen by delivering during non-therapeutic periods of the respiratory cycle.
FIG. 1 further provides a graph of the flow rate of oxygen by the HELiOS® H300 LOX system as waveform 115. As shown by waveform 115, the HELiOS® H300 LOX system begins delivering oxygen before the other two units at around 0.2 seconds after the beginning of inspiration. Furthermore, the HELiOS® H300 LOX system delivers a relatively large impulse of oxygen in a relatively short period in comparison to the other pneumatic devices shown in FIG. 1. More particularly, the HELiOS® H300 LOX system delivers oxygen at around 19 SLPM for an initial short impulse period and then tapers off to a lower flow rate of oxygen. For the particular delivery cycle shown by waveform 115, the HELiOS® H300 LOX system does, however, continue to deliver oxygen at around 1 SPLM for a time period after the therapeutic cutoff of 0.5 seconds.
The HELiOS® H300 LOX system, like the majority of dual lumen systems, continues to provide oxygen until the patient exhales. Therefore, the HELiOS® H300 LOX system will waste gas until the patient begins to exhale. As illustrated by the example cycle shown in waveform 115, the HELiOS® H300 LOX system continues to deliver oxygen until around 0.6 seconds. Thus, while demonstrating superior flow dynamics to that of the Easymate Liquid Oxygen System and the Chad Cypress oxygen delivery system, the HELiOS® H300 LOX system wastes oxygen by delivering during non-therapeutic periods of the respiratory cycle.
An additional waveform 120, shown on FIG. 1, illustrates a compressed oxygen system (high pressure tank system) employing an electronic oxygen conserver device, namely the Respironics' electronic Pulse Oxygen Device (ePOD™). This device is disclosed in U.S. patent application Ser. No. 11/096,993 (publication no. 2006 0219245). The ePOD system with an electronic oxygen conserver device has a distinctly different pulse image, shown by waveform 120, than those of the pneumatic oxygen conserving devices of waveforms 105, 110, and 115. The pulse flow of the ePOD electronic oxygen conserving device begins relatively quickly, around 0.15 seconds after the beginning of inspiration, and ends around 0.4 seconds. Waveform 120 illustrates that the electronic oxygen conserving device delivers a consistent burst of oxygen at around 11 SLPM for around 0.12 seconds and then quickly tapers off. Furthermore, waveform 120 illustrates that the compressed oxygen system employing an electronic oxygen conserving device does not deliver oxygen after a half second from inspiration.
The pulse of the electronic oxygen conserving device can be described as a pulse dose or box pulse, in that a relatively large amount of oxygen is delivered by a flow dynamic that starts and stops abruptly. Therefore, some electronic oxygen conserving devices can be relatively successful at delivering oxygen only during the therapeutic period of the respiratory cycle.
As shown by FIG. 1, conventional oxygen delivery systems with conventional pneumatic conserving devices, while suitable for their intended purposes, are insufficient at preventing oxygen waste. As shown by FIG. 1, the pulse dose characteristics of the oxygen delivery systems with an electronic oxygen conserving device provide an oxygen delivery pulse that is superior to that of the conventional oxygen delivery systems with pneumatic conserving devices shown in FIG. 1. As discussed above, however, oxygen delivery systems with electronic oxygen conserving devices suffer from many of their own drawbacks, relating to power consumption and complexity, among other issues.
Accordingly, a need exists for a liquid oxygen delivery system incorporating a pneumatic oxygen conserver capable of matching a patient's needs for oxygen as closely as possible. Additionally, a need exists for a pneumatic based oxygen delivery system capable of providing an oxygen delivery pulse similar to that of an efficient oxygen delivery system incorporating an electronic oxygen conserving device. In other words, a need exists for a pneumatic conserver system capable of mimicking the oxygen delivery of an electronic conserver system. Furthermore, a need exists for a oxygen delivery system capable of overcoming the drawbacks of both conventional pneumatic conserving device oxygen delivery systems and electronic oxygen conserving device oxygen delivery systems.